This invention relates to repair or refurbishment of a metallic coating including a surface oxide grown from at least one element of the coating as a result of exposure of the metallic coating to oxidizing conditions at an elevated temperature. More particularly in one form, it relates to a metallic coating including the element Al on a metallic article, in one specific form including a substantially uncoated article portion, for example a gas turbine engine blading member including a substantially uncoated radially inward blade base portion.
In the development of certain components operating in the hotter sections of modem gas turbine engines, it had been recognized that structural metal alloy materials from which such components are made alone are unable effectively to resist surface deterioration from the strenuous operating conditions, even with air cooling capability. For example, a high temperature environment to which the component surface is exposed includes oxygen and products of fuel combustion as well as airborne particles. As a result, a variety of types of surface protective coatings have been developed and reported for commercial application to such components, generally made from a mechanically strong superalloy based on at least one of Fe, Co, and Ni.
A gas turbine engine turbine blade made of a commercially available Ni base superalloy is a typical example of such a component. It has become common practice to protect the blade surface exposed during service operation to the strenuous environmental conditions with a metallic coating including the element Al. A wide variety of such metallic coatings have been reported and used on production gas turbine engine components including shrouds, bands, and blading members such as rotating blades, and stationary blades, vanes and struts. Such commercial coatings include diffused aluminides, a commercial form of which sometimes is called Codep aluminide coating, deposited by such diffusion deposition methods as pack cementation, within or above a pack, by vapor phase aluminiding, etc. Another of such metallic coatings is the Ptxe2x80x94Al type coating in which Pt first is deposited, such as by electrodeposition, on a surface that subsequently is diffusion aluminided. Still another type of such metallic coating is a metallic overlay coating of the Mxe2x80x94Al type in which M is at least one element selected from Fe, Co, and Ni, for example MAl, MAlY, MCrAl, and MCrAlY. The Mxe2x80x94Al types of coating can be applied by such methods as physical vapor deposition, including sputtering, cathodic arc, electron beam, and plasma spray. Sometimes such coatings including Al are not used as an outer protective coating but have been used as an intermediate or bond coat beneath an outer non-metallic ceramic thermal barrier coating disposed over the coating including Al.
When a metallic coating including Al, for example used as the outer coating for a turbine engine component, is exposed to the above described type of strenuous service operating conditions, aluminum oxide is grown thermally at the component outer surface from Al in the coating. Such generation of the oxide depletes Al from the coating and can reduce the protective capability of the coating. This is particularly significant with the above described Mxe2x80x94Al type overlay coating that generally includes less Al, for example in the range of about 10-20 weight %, than the diffusion aluminide coatings. Formation of surface aluminum oxide from an overlay coating can reduce the Al content to less than about 10 wt. %, typically to the undesirable range of about 5-10 wt. %. During repair of a turbine engine component from service operated damage or as a result of excessive Al depletion from the protective coating, it is necessary to remove the surface thermally grown oxides to enable repair and/or coating refurbishment or replacement.
Reported methods for removal of the surface oxide include use of a halogen ion, for example fluoride ion alone or in combination with a reducing gas such as hydrogen, to convert the oxide to a halide vapor. Other methods include use of abrasive blasting or mechanical means such as machining or grinding, that removes at least a portion of the metallic coating as well as the oxide. Another method includes the use of chemical solutions such as relatively strong caustics and/or acids to remove the oxide and the coating. However, some components, for example gas turbine engine rotating turbine blades, typically include a portion at least on the radially inner surface of the blade base, which has no need for and generally does not include a protective coating. It has been observed that use of such known methods involving halide ion and relatively strong chemical solutions can result in undesirable intergranular attack of such uncoated surface.
The present invention, in one form, provides a method for refurbishing a service operated metallic coating, for example the above described type of metallic overlay coating, on a substrate alloy surface. The service operated coating includes at least within a coating outer surface at least one oxide, for example aluminum oxide, chemically grown from at least one coating element, for example Al, and chemically bonded with the coating outer surface as a result of thermal exposure during service operation. Growth of the oxide depletes at least a portion of the coating element from the coating.
The method comprises removing the chemically grown oxide from the coating outer surface by a means which substantially only affects the oxide and does not affect the underlying coating or an exposed substrate alloy surface. For example, such removal can be mechanically by a controlled relatively light grit blasting and/or a relatively weak acid solution such as acetic acid. The metallic coating depleted, during operation, of at least a portion of the coating element, for example Al, substantially is retained during such oxide removal. This action exposes in the coating surface at least one surface void that had been occupied by the oxide. If the oxide extends substantially across the coating surface, the exposed void or voids appear as a roughened surface.
The retained metallic coating surface with the exposed void or voids is mechanically worked such as by impacting, rather than being abraded, for example mechanically worked by a commercial tumbling method, substantially without removing the retained coating. Such working closes the void, and provides a coating surface finish of no greater than about 60 microinch Roughness Average (RA). Concurrently, the working provides a compressive stress in the substrate surface and the coating. This provides a treated metallic coating outer surface over which a refurbishing coating is applied.